Extensible Object Location System and Method using Multiple References

ABSTRACT

An example disclosed method includes defining a first zone within a monitored area, a first group of receivers covering the first zone; defining a second zone within the monitored area, a second group of receivers covering the second zone; determining, via a processor, a first position of a first tag based on timing measurements obtained via the first group of receivers; determining, via the processor, whether the first position indicates that the first tag is within the first zone; and when the first position indicates that the first tag is not within the first zone, not reporting data associated with the first tag.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent arises from a continuation of U.S. patent application Ser.No. 13/436,029, filed Mar. 30, 2012, which is a continuation of U.S.patent application Ser. No. 12/769,105, filed Apr. 28, 2010, now U.S.Pat. No. 8,149,169, which is a continuation of U.S. patent applicationSer. No. 11/331,229, filed on Jan. 13, 2006, now U.S. Pat. No.7,710,322, which claims the benefit of U.S. Provisional PatentApplication No. 60/679,235, filed May 10, 2005. U.S. patent applicationSer. Nos. 13/436,029, 12/769,105, and 11/331,229 and U.S. ProvisionalPatent Application No. 60/679,235 are hereby incorporated herein byreference in their entireties.

BACKGROUND

This invention relates to a radio frequency (“RF”) object locationmethod and apparatus. More specifically, this invention relates tosystem architecture, as well as an improved calibration method andapparatus for precisely locating an object(s) in an arbitrarily large,physically connected or disconnected multipath, and/or noisyenvironment.

RF location systems are used to keep track of objects such as inventory,materiel, equipment, personnel, or other items. In such systems, objectsto be located typically have associated transmitters or transponders,commonly referred to as active RF tags. To locate the object, varioustechniques have previously been used to process received signals.

In prior systems, RF sensors (also referred to as “monitoring stations”)were positioned at known coordinates within and/or about an area to bemonitored. RF emissions from associated object tags were received andprocessed by these sensors. Signal processing schemes included measuringrelative signal strength, angle of arrival (AOA), or time differences ofarrival (TDOA or DTOA) at the respective sensors. Typically, systemsbased upon TDOA determined differences in the arrival time of the signalfrom the tag at one monitoring station relative to other monitoringstations. Measurement of the time difference was often accomplishedusing a digital counter whose count was latched in response to receiptof an incoming RF signal. Systems based upon such TDOA measurements weresometimes referred to as “multilateration” or “geolocation” systems,which refer to the process of locating a signal source by solving forthe mathematical intersection of multiple hyperbolae, determined by thedifferences of arrival times between signals received at multiplesensors.

In another class of prior systems described, for example, in U.S. Pat.No. 4,740,792 and commonly-owned, incorporated U.S. Pat. No. 6,054,950,untethered monitoring stations relayed received signals via wirelesslinks to a central measurement unit. Although well-suited for monitoringobject locations in large outdoor areas, or in applications where wiringwas not feasible or too expensive to install, this approach required atransmitter and receiver at each station.

In another class of prior systems (cf. U.S. Pat. Nos. 3,714,573;5,216,429; 5,920,287; and 6,121,926), tethered monitoring stationsrelayed radio frequency signals via cables to a central measuring unit.One drawback of this approach was signal dispersion in the cable.Generally, dispersion is a process by which an electromagnetic signalpropagating in a physical medium becomes degraded due to various wavecomponents, or frequencies, of the signal propagating at differentvelocities within the medium. Dispersion reduces the edge-rate orrise/fall times of the signals thereby degrading the ability of thesystem to accurately detect arrival time, and hence, to determine theposition of the object.

In yet another class of prior systems (cf. U.S. Pat. Nos. 3,419,865;3,680,121; and 4,916,455), measurement schemes were implemented at eachof the monitoring stations to produce a digital result indicative ofarrival times, angle of arrival, or other value. Advantageously, thesesystems conveyed digital data via interconnecting cables; and hence,position accuracy was not affected by cable dispersion. However, adrawback of this approach relates to the fact that these systems areplesiochronous, or “nearly” synchronous; i.e., timing reference signalswere arbitrarily close in frequency (within some specified limits), butwere not sourced from the same clock signal. Thus, over some period oftime, the timing reference signals drifted with respect to each other.As each monitoring station had an independent clock source, smalldifferences in clock frequencies degraded accuracy in positionmeasurement, which worsened over time.

Yet another class of prior systems included synchronous systems, i.e.,those in which the timing reference signals were derived from a commonsource. In some synchronous systems (cf. U.S. Pat. Nos. 5,317,323 and6,094,169), a local timing reference clock was derived from a GlobalPositioning Satellite (GPS) timing source. While this was suitable forfrequency synchronization in benign outdoor conditions, monitoringstations operating indoors or in urban environments could not generallyrely upon receiving a GPS timing signal, and consequently, objectlocation could not be determined.

U.S. Pat. No. 5,119,104, for example, describes a synchronization schemein which a timing reference clock was provided at each monitoringstation receiver by way of a local area network (LAN) cable. At eachmonitoring station, the clock signal incremented a digital counter thatlatched a count value upon receiving an RF signal arriving at anassociated receiver of the monitoring station. Advantageously, thisparticular approach guaranteed that all counters operate at the sameclock frequency. However, a drawback was the lack of a provision toreset the counters or otherwise control the relative phase between them.Non-compensated phase offset between counters degraded positionaccuracy. Furthermore, in the system described in the '104 patent, themonitoring stations included a data communication controller thatresponded to the receipt of an object tag transmission and, upon receiptthereof, sent a corresponding time of arrival (TOA) detection packet toa centrally located processor. In other words, such system wasinterrupt-driven where receipt of a tag transmission signal invoked aninterrupt. A drawback of this approach was that, upon receiving a firsttag transmission, the system was temporarily “disarmed” and thus unableto process a second tag transmission until the network completed thetransfer of measurement data. Thus, it was possible that one or more tagtransmissions were lost in the process.

Phase offset between counters among the respective monitoring stationscan be controlled by a synchronizing or counter reset signal. U.S. Pat.Nos. 3,680,121 and 4,916,455, for example, disclose object locationsystems utilizing an RF synchronizing signal that was transmitted toeach monitoring station in the monitored region. To avoid interference,the synchronizing signal was transmitted at a frequency distinct fromthat of the tag transmission. Thus, one drawback of this approach wasthat each monitoring station had to be equipped with two distinct RFreceivers—a first to sense the tag transmission and a second to sensethe synchronization signal. Alternatively, the system disclosed by U.S.Pat. No. 3,419,865 included a cable interconnecting a central unit andeach monitoring station to enable “adjusting their time clocks toprecise mutual synchronization.” A drawback of this approach, however,was signal dispersion in the cable, which reduced pulse sharpness andtiming accuracy of the synchronizing signal.

Synchronizing or calibration methods applicable to radio frequencylocation systems are also known (cf. U.S. Pat. Nos. 4,916,455;5,119,104; and 6,094,169). A general synopsis of a calibration techniqueis provided in the '455 patent, in which it is stated that “[i]n orderto achieve the high accuracy, the system was periodically calibrated.System calibration was accomplished by periodically transmitting amodulated signal (with a unique calibration identity code) from a knownlocation. The transition times of arrival derived therefrom were thentransmitted to a central analyzer for time-difference processing. Theresulting time differences were then compared to known values and errormagnitudes were then used to compensate corresponding station-pair timedifferences resulting from other unknown-location transmissions.”

The need for calibration is also summarized in the '104 patent asfollows. “To operate the radiolocation system with TOA resolution innanoseconds, minute changes in circuit operational parameters and signalpropagation characteristics, such as might result from changes intemperature and humidity within the facility, had to be taken intoaccount. Such changes were accommodated through system calibration”.

Another problem unique to determining object location or to track assetsis that, in order to accurately determine position, a minimum number ofreceivers at the monitoring stations (i.e., typically three receivers)must have a direct (i.e., a line-of-sight or, at most, an attenuatedline-of-sight) transmission path. However, due to the nature of indoorenvironments, there may only be a limited number of such directtransmission paths. For example, walls, machinery, containers, and othermaterials may create signal attenuation or even complete signalblockage. Thus, there may exist certain zones within the monitoredregion in which position accuracy may be degraded for lack of adequatesignal reception. A solution to this problem was to provide redundantmonitoring stations. However, in providing such redundancy, it becomespossible, and in fact likely, that more than the minimum number ofmonitoring stations will receive a given transmission. Such a system isoften referred to as an “over-specified” or “over-determined” system.

A potential drawback of using an over-determined system relates to thefact that hyperbolic ranging algorithms can calculate more than onemathematically valid position. That is, ambiguities in positiondetermination can arise. Various techniques have been applied to addressthis issue. For example, U.S. Pat. No. 5,166,694 discloses a method ofcomputing a vehicle location in an overdetermined system. One aspect ofthe '694 patent is the use of a pre-filter to “remove any signals thatwere corrupted by anomalies in the propagation of the transmittedsignal.” In particular, the specification thereof describes a “multipathfeasibility circle” that is determined by a system parameter that is anestimated maximum speed of the vehicle containing the transmitter. Adrawback of this approach is that it is possible for a signal to have apropagation anomaly and yet not produce an error sufficiently largeenough to be rejected or filtered out.

In commonly-owned, incorporated U.S. Pat. No. 6,882,315, many of theabove noted problems were resolved, and highly accurate (e.g., +/−1 footor better) position measurements were obtained using a measurementapparatus utilizing ultra wideband (UWB) signals disposed at each of themonitoring stations; a timing reference clock to synchronize thefrequency of counters within respective monitoring stations; and areference transmitter positioned at known coordinates to enable accuratedetermination of the phase offset between counters.

In the '315 patent, a single reference tag transmitter enabled precisedetermination of phase offsets for a given set of monitoring stations. Asingle-reference tag system, however, had limitations in certainsituations.

First, the reference signal from single reference transmitter must bereceived at each and every one of the monitoring stations. Given thepeak and average power constraints imposed upon all licensed andunlicensed transmitting devices by the Federal Communications Commission(FCC), there is a maximum range over which reception can be reliablyachieved. For example, in one embodiment of a UWB tracking system, thereference tag transmission is capable of being reliably receivedoutdoors at ranges of approximately 650 feet, and indoors (dependingupon obstructions) at ranges of approximately 200 to 300 feet. With aworst case range of 200 feet from reference tag to monitoring station(UWB receiver), a single reference tag system has a maximum coveragearea of approximately 40,000 square feet, which may not be adequate forcertain applications.

Secondly, as noted above, the single reference tag must be placed at afixed site having direct transmission paths to all of the individualmonitoring stations. In indoor applications, this can be quitechallenging, and sometimes impossible, when obstructions (e.g.,steel-reinforced concrete walls, machinery, metal doors, etc.) createsignificant signal attenuation or even complete signal blockage.

While one solution to such limitations would be to replicate the objectlocation system with a new reference transmitter supplied for eachreplication, cost constraints ultimately limit the benefit of such asimple approach. For example, this approach would typically requiresignificantly more receivers than necessary for area coverage.Furthermore, there are numerous implementation geometries (describedfurther below) in which a single reference tag may not be sufficient toovercome signal blockages, resulting in a system with either increasedmeasurement inaccuracies or dead zones in which no positional data canbe extracted.

It is thus desirable to have a precision object location system ormethod capable of monitoring large areas (e.g., hundreds of thousands tomillions of square feet), while offering flexibility to overcome signalblockages extant in realistic environments, and that may use a reducedset of receivers for complete area coverage.

In view of the foregoing, it is a feature of the present invention toprovide highly accurate position measurements (e.g., +1/−1 foot orbetter) by providing a measurement apparatus or method, preferably usingUWB signals, that is operable at each of the monitoring stations; atiming reference clock to synchronize respective frequency of counters(or other timers) within the monitoring stations; and one or morereference transmitters, preferably UWB transmitters, positioned at knowncoordinates within a monitored region to enable accurate determinationof phase offsets between respective timers or counters of the monitoringstations.

It is another feature of the present invention to use a multiplereference tag algorithm and virtual group, or zoning, technique thatpermits geolocation of tags over multiple monitored areas, where theareas are contiguous (e.g., in a mosaic pattern), overlapping, or evenfully separated by some distance. Such a system affords much greatercoverage and system scalability, while maintaining the high positioningaccuracy achievable with a single reference system.

In commonly-owned U.S. Pat. Nos. 6,054,950 and 6,882,315 referencedabove, ultra wideband (UWB) waveforms were employed to achieve extremelyfine, centimeter-type resolution because of their extremely short (i.e.,subnanosecond to nanosecond) durations. This invention also utilizesUWB, or short pulse, technologies to provide an improved object locatingsystem and method for asset tracking that addresses the above-mentionedshortcomings of prior systems. The apparatuses and methods identifiedherein are equally applicable to wideband pulse and spread spectrum RFtechnologies with some sacrifice in position accuracy.

Other aspects, features, and advantages of the invention will becomeapparent upon review of the succeeding description taken in connectionwith the accompanying drawings. The invention, though, is pointed outwith particularity by the appended claims.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided asystem to determine a position of an object in any one of multiple zonesof a region where the object has an associated object tag that transmitsan RF signal. Such an embodiment comprises a receiver group in each zonethat includes multiple receivers wherein at least one of the receiversof one zone is included in a receiver group of at least one other zone.A cyclic timer associated with each receiver detects an arrival time ofan RF signal, which may be an ultra wideband signal. A common clocksource synchronizes the frequency of cyclic timers of each receiverthereby to produce phase offsets between cyclic timers of the differentreceivers of the group. A reference tag associated with each receivergroup transmits an RF reference signal (which may also be an ultrawideband signal) from a known location to enable determination of timingoffsets. A processing hub in communication with the receivers determinesthe position of the object according to timing offsets and timedifferences of arrival of RF signals transmitted by the object tag.

In a further embodiment, the common clock source includes adaisy-chained cable to convey clock signals to the receivers and theprocessing hub includes a data cable to convey digital representationsof the time differences of arrival of said RF signals. In yet a furtherembodiment, the cyclic timers comprise digital counters that aresynchronized in frequency, but not necessarily in phase, by the commonclock source. Optionally, the receiver groups may be arranged in apattern to define a matrix of rows and columns of zones. Alternatively,the receivers and reference tags may be arranged substantially in a ringconfiguration to encircle a region of interest within which to detect anobject having an associated object tag. The reference tags may alsotransmit an ID code to distinguish among multiple reference tags whendetermining object location according to time difference of arrival.Further, the processor may determine offsets of timers in a groupaccording to offsets of other timers in a contiguous group having ashared receiver.

In accordance with another embodiment of the invention, there isprovided a zone-based object location system to determine a position ofan object in one of multiple zones of a region that comprises respectivegroups of receivers that respectively define zones where at least onereceiver is common to at least two zones. Each receiver has a cyclictimer (e.g., free-running digital counter) to measure an arrival time ofan RF signal (e.g., a wideband or ultra wideband signal). A clock source(preferably common to all receivers) locks the phase relations of thetimers relative to each other and a reference tag associated with eachthe zones transmits an RF signal from a known location of the zones. Aprocessing hub obtains time-of-arrival information indicative of arrivaltimes of RF signals transmitted by the object and reference tags atreceivers of a receiver group whereby to determine the position of theobject within a zone. Optionally, the processor may obtain digitalrepresentations of time-of-arrival information via a digital data cable.

In yet a further embodiment of the invention, there is provided azone-based object location system to determine a position of an objectin one of multiple zones of a region comprising receiver groups thatdefine said zone where each receiver group includes multiple receiversand each receiver includes a free-running digital counter to measure anarrival time of an ultra wideband signal, a common clock to lock thecounters in relative phase relationship via a daisy-chained cable, areference tag associated with each zone to transmit an ultra widebandsignal from a known location in order to provide a timing reference, anda processing hub to obtain digital representations of time-of-arrivalinformation indicative of the timing reference and arrival times of theultra wideband signals transmitted by the object tag whereby todetermine the position of the object.

In yet another embodiment of the invention, there is provided anextensible zone-based object location system to determine a position ofan object in any one of multiple zones of a region comprising respectivegroups of receivers that define respective zones where at least twozones share a receiver and where each receiver includes a timer tomeasure an arrival time of an RF signal, a reference tag associated witheach zone to transmit an RF signal from a known location, and aprocessing hub to obtain time-of-arrival information indicative ofarrival times of RF signals transmitted by said object tag and referencetag whereby to determine the position of the object within said zone.

A further embodiment of the invention comprises a method of determiningthe position of an object having an associated object tag that transmitsan RF signal. This embodiment comprises the steps of providingrespective groups of receivers to define respective zones in a monitoredregion in which to detect said object wherein at least two zones share areceiver, transmitting a reference signal from a fixed location relativeto a group of receivers of a zone, receiving the reference signal ateach receiver of said group of receivers to determine a timing offsetamong the receivers of the group, transmitting an object tag signalwithin the zone, receiving the object tag signal at the receivers of agroup of receivers, and computing object location based on offsets andtime of arrival differences of the object tag signal at multiplereceivers within said zone. When cyclic timers are used to measureoffsets, they may comprise free-running counters and said receiving stepmay further include indexing the cyclic timers by a common clock sourceto produce unitary increments of time indicative of said timing offset.Further, the method may include arranging the zones to define around RFbarriers within a monitored region and/or the determining step mayutilize time offset information to discriminate against reflectedsignals within a zone.

In yet a further aspect of the invention, an extensible object locationmethod to determine the position of an object having an associatedobject tag that transmits an ultra wideband signal comprises providingrespective groups of receivers to define respective zones of a monitoredregion in which to detect the object wherein at least two groups share acommon receiver, providing a reference tag transmitter to transmit areference signal from a known location with respect to each zone,receiving the reference signal at each receiver of a zone thereby todetermine a timing offset for the receivers of the zone, transmitting aobject tag signal within the zone, receiving the object tag signal ateach receiver of said zone, and determining the position of said objectbased on the timing offsets among receivers and differences in time ofarrival of the object tag signal at the receivers within the zone.

A further embodiment comprises an extensible object location method todetermine the position of an object having an associated object tag thattransmits an RF signal wherein the method comprises providing successivereceivers to at least partially encircle a monitored region in which todetect an object, providing a reference tag transmitter substantiallybetween each of the receivers to transmit a reference signal from aknown location, receiving the reference signal at at least two receiversthereby to determine timing offsets for at least two receivers,transmitting an object tag signal from the monitored region, receivingthe object tag signal at the at least two receivers, and determining theposition of the object based on the timing offsets and differences intime of arrival of the object tag signal at the at least two receivers.

Other aspects and embodiments will become apparent upon review of thefollowing description taken in conjunction the accompanying drawings.The invention, though, is pointed out with particularity by the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 illustrates an object location system having receivers arrangedin a daisy-chain interconnection.

FIG. 2 illustrates an object location system having receivers configuredusing a combination of star and daisy-chain interconnections.

FIG. 3 shows an example of an object location system according to thepresent invention where multiple reference tags are deployed inrespective zones of a monitored region.

FIG. 4 illustrates another aspect of an object location system accordingto the present invention where coverage is expanded using multiplereference tags in multiple zones of a monitored region.

FIG. 5 illustrates how the use of multiple reference tags according tothe present invention may overcome signal blockage.

FIG. 6 illustrates how a bounced signal can cause a faulty TDOAmeasurement at a particular receiver.

FIG. 7 shows a system configuration according to another aspect of thepresent invention in which multiple reference tags are position outsidea monitored region.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 illustrates a prior radio frequency (RF) object locating systemutilizing a single fixed reference tag transmitter that providestime-of-arrival reference information to enable a processor to determinethe position of an object in a monitored region generally locatedbetween and about a number of monitoring stations 100 a, 100 b, 100 c,and 100 d. A stationary or mobile object 99 to be located has anassociated or co-located object tag transmitter 102, such as a UWB orwideband transmitter, that transmits a short packet burst for TOA orDTOA timing, and optionally, an information packet that may include, butnot be limited to, identification information (ID) information and/or asequential burst count specifying a sequence number of the transmittedburst when multiple bursts are transmitted. At least one othertransmitter, depicted as reference transmitter 103, is positioned withinand/or about the monitored region.

In FIG. 1, one or more (preferable three or more) monitoring stations100 a, 100 b, 100 c, and 100 d being substantially identical or similarin structure and/or function are also positioned at predeterminedcoordinates within and/or around the monitored region. The monitoringstations detect signals transmitted by the object tag 102 and thereference tag transmitter 103. Each of the monitoring stations 100 a-100d includes a receiver that receives tag transmissions (preferably UWBtransmissions); and preferably, a packet decoding circuit that extractsa TOA timing pulse train, transmitter ID, packet number and/or otherinformation that may have been encoded in the tag transmissions (e.g.,materiel description, personnel information, etc.).

In addition, each of the monitoring stations 100 a-100 d includes atimer or time measuring circuit to measure or detect the arrival time ofa tag transmission. The time measuring circuit is frequency-locked witha common, e.g., digital, reference clock signal distributed via cablefrom a central timing reference clock generator, preferably containedwithin Processing Hub 101. Thus, multiple time measuring circuits of therespective monitoring stations 100 a, 100 b, 100 c, and 100 d aresynchronized in frequency, but not necessarily in phase. While theretypically may be a phase offset between any given pair of receivers inthe monitoring stations, the offset is readily determined through use ofreference transmitter 103, as described in the commonly-owned,incorporated U.S. Pat. No. 6,882,315.

FIG. 1 shows a “daisy chained” architecture in which a Processing Hub101 feeds power and clock signals to UWB Receivers 100 a-100 d in themonitoring stations in a serial fashion via a Power and Clock Cable 104.Each UWB Receiver, in turn, forwards the power and clock signals to thenext UWB receiver in a serial chain. Similarly, time-of-arrival and datafrom the UWB Receivers is sent back in serial fashion through themonitoring stations 100 a-100 d to the Processing Hub 101 via a SerialData Interface Cable 105. In one embodiment, the Power and Clock Cable104 and the Serial Data Interface Cable 105 are simply separate wireswithin a single cable, e.g., CAT-5 or CAT-6 local area network cable.

FIG. 2 shows an alternative prior art embodiment of FIG. 1 in which theProcessing Hub 101 and UWB receivers in the monitoring stations form twodistinct serial chains comprising stations 100 a and 100 b in a firstchain, and stations 100 c and 100 d in a second chain. Receivers #1 and#2 form part of the first chain while Receivers #3 and #4 form part ofthe second. While only two distinct chains are shown, the arrangement isreadily extensible to an arbitrary number of distinct serial chains ofreceivers. Such a “star” or “hub and spoke” architecture providesconsiderably more flexibility for system installation and eliminates thesingle-point failure mode of a serial “daisy chained” architecture ofFIG. 1. However, with only a single reference tag 103, placement of theUWB receivers still may be constrained by the requirement that each ofthe receivers 100 a-100 d receive direct line-of-sight transmissionsfrom reference tag transmitter 103, which may significantly limit thecoverage.

As described in the commonly-owned U.S. Pat. No. 6,882,315, a referencetag transmitter synchronizes the receiver's timing devices (e.g., delaylines, free-running digital counters, etc. that latch or record countvalues representing unitary increments of time defined by delay linetaps or counter frequency at which the RF signal is sensed or detected)in order to provide time-of-arrival information from each target orobject tag associated with the object(s) being located. The referencetag transmitter is positioned to be clearly visible via line-of-sight toall receivers. A typical installation, for example, would have thereference tag transmitter located roughly at the center of the area ofcoverage or the monitored region.

In an improved object location system or method according to an aspectof the present invention, however, the system or method involvesdeployment of multiple reference tag transmitters (herein called“reference tags”) so as to cover multiple distinct areas or zonesdefined in the monitored region by the monitoring stations. Asillustrated in FIG. 3, these areas or zones 110 and 112 may beoverlapped, or annexed to each other, or even separated by a distance.FIG. 3 exemplifies a multi-reference mode of deployment in which tworeference tags 114 and 116 are respectively placed in the two separatezones 110 and 112. Zone 110 includes a reference tag 114 as well asreceivers #1, #2, #3, and #4 respectively located monitoring stations100 a, 100 b, 100 c, and 100 d. Zone 112 includes a reference tag 116 aswell as receivers #3, #4, #5, and #6 respectively located in monitoringstations 100 c, 100 d, 100 e, and 100 f. In this example, receivers #3and #4 are effectively shared by zones 110 and 112 to monitor both zones110 and 112. Similar to the single reference tag mode, all tag datacaptured by the receivers are transferred to the central processing hubunit 118 for processing to compute object location. Software inprocessing hub 118 implements a multi-reference mode algorithm, which issubsequently described.

A processor in the central processing hub uses digital representationsof time differences of arrival (TDOA) of the tag transmissions receivedat the various monitoring stations to determine object location. Theobject may be fixed or mobile. Positioning is achieved by performing agradient search (or mathematical equivalent search) for the optimal tagposition (having unknown coordinates x, y and z) to minimize thefollowing error function:

$\begin{matrix}{ɛ = {\sum\limits_{i = 1}^{N}\left\lbrack {\sqrt{\left( {x - x_{i}} \right)^{2} + \left( {y - y_{i}} \right)^{2} + \left( {z - z_{i}} \right)^{2}} - {c\left( {t_{i} - t_{0}} \right)}} \right\rbrack^{2}}} & (1)\end{matrix}$

where N is the number of receivers at monitoring stations, c is thespeed of light, x_(i), y_(i) and z_(i) are the coordinates of eachi^(th) receiver, and t_(i) is the arrival time of an object tagtransmission (which may also include a message) at the i^(th) receiver.The unknown dummy variable t₀ represents a transmission time of the tagtransmission in an absolute time epoch. The optimization proceeds, forexample, through the use of a non-linear optimization technique such asthe Davidon-Fletcher-Powell (DFP) quasi-Newton algorithm. Since thereare four variables (x, y, z, t₀) in the error function c, a minimum offour receivers are required in order to unambiguously determine thethree-dimensional (3D) position of an object tag. To resolve atwo-dimensional (2D) position, at least three receivers are required;while for one-dimensional positioning, tag transmission data from onlytwo receivers is necessary.

From equation (1), it is easy to observe that the time difference ofarrival (TDOA), rather than the absolute time of flight (TOF), affectsthe object tag position result (x, y, z) within the optimizationprocess. For example, if an arbitrary time offset is added to eacharrival time t_(i) (i.e., t_(i)→(t_(i)+t) for i=1 to N), the effectwould be equivalent to setting the unknown temporal parameter t₀→(t₀−t).In other words, the net result would be the same for the solution of theobject tag coordinates (x, y, z).

Thus, without loss of generality, the error function can be re-writtenas

$\begin{matrix}{ɛ = {\sum\limits_{i = 1}^{N}\left\lbrack {\sqrt{\left( {x - x_{i}} \right)^{2} + \left( {y - y_{i}} \right)^{2} + \left( {z - z_{i}} \right)^{2}} - {c\left( {{\Delta \; t_{ij}} - t_{0}} \right)}} \right\rbrack^{2}}} & (2)\end{matrix}$

where Δt_(ij) is the difference in the time of arrival for a tagtransmission received by receiver i and j, where j is any index from 1to N. That is, j denotes a common receiver ID from which all TDOA dataare measured with respect to.

In order to determine the values of Δt_(ij), one or more reference tagsare required to calibrate counter (or timing) offsets between differentreceivers. Exemplary counters in the receivers run at same clockfrequency since they have the same clock source (e.g., sent by cable 104from the hub 118); however, the counters could start at different timesand hence likely have very different offsets. Receiver clocks instations 100 a-100 f are not typically phase-locked with a clock of anyother receiver in the monitoring stations. Hence, an additional counteroffset could be induced by clock skew due to different cable lengths andunmatched internal delays within the receiver clock circuits.

Let ΔR_(ij) be the counter offset between receivers i and j. Assume thata reference tag pulse transmission is captured by each of receivers iand j with time counter values of C_(i)(ref) and C_(j)(ref),respectively. Counter offset may then be determined by the relationship:

ΔR _(ij) =└C _(j)(ref)−D _(j)(ref)/c┘−[C _(i)(ref)−D _(i)(ref)/c]  (3)

where D_(i)(ref) and D_(j)(ref) represent the distances from thereference tag to receivers i and j, respectively. Since one knows eachreference tag location (x_(ref), y_(ref), z_(ref)) as well as thecoordinates of each receiver, these distances may be very accuratelydetermined a priori.

It is important to note that the counter offset ΔR_(ij) will be heldconstant as long as the system runs without any power interruption tothe receivers. This is due to the fact that the receivers in theillustrated embodiment share the same clock source and use exactly thesame clock frequency for incrementing their respective time counters.Instead of using counters as timing devices, alternative timing circuitsor methods a may come to those skilled in the art based on the teachingsherein may also be effectuated to achieve the same or similar result.Thus, the invention is not limited to digital counters as a means tomeasure elapsed time. In addition, instead of using a Cartesiancoordinate system as illustrated by equations (1) and (2), processing todetermine object location may be accomplished using other geometric orcoordinate representations of position, such as spherical polarcoordinates or cylindrical polar coordinates. Moreover, certain aspectsof the invention are not limited to using timer offsets to computerobject location but instead embraces geolocation systems and methods,generally.

A transmission signal burst from a reference tag then produces onemeasurement of the quantity ΔR_(ij), and periodic bursts from the same,or even different reference tags, provide continuously updatedcorrections to these ΔR_(ij) measurements. A determination of the valuefor ΔR_(ij) can thus be achieved with sub-nanosecond accuracy, providingthe basis to obtain sub-foot resolutions of tag positions.

Before starting the optimization process to compute object tag position,a complete set of timing offsets {ΔR_(ij)} between the receivers (whichare participating in a particular position computation) must first bemeasured or determined. Note that ΔR_(ii)=0 and ΔR_(ij)=−ΔR_(ji) for alli and j indices so that only N(N−1)/2 measurements and computations arerequired.

Once the timing offset values are available, the time differences ofarrival {Δt_(ij)} for an object tag can be calculated from thetime-stamp measurements at the receivers as follows:

Δt _(ij) =[C _(j)(tag)−C _(i)(tag)]−ΔR _(ij).  (4)

where C_(i)(tag) and C_(i)(tag) are, in the illustrated embodiment, timecounter values of the object tag burst captured at receivers i and j,respectively. From a set of N such count values (i=1 to N and j anyfixed index), the Davidon-Fletcher-Powell (DFP) algorithm, orequivalent, is used to minimize the error function (eq. 2) which yieldsan optimal estimate for the true object tag position (x, y, z).

With a single reference tag, as noted above, all receivers whose timecounter values used in the optimization procedure must have a directline-of-sight path to the reference tag. However, with multiplereference tags as shown in FIG. 3, multiple “reference groups” aredefined for the respective zones 110 and 112, with each group within azone comprising a reference tag, e.g., tag 114, and a group of receiversassigned to that specific reference tag, e.g., receivers #1, #2, #3, and#4 of monitoring stations 100 a, 100 b, 100 c, and 100 d. The referencegroup is so configured such that each receiver within the group has adirect line-of-sight path to its corresponding reference tag. In thiscase, counter offsets are measured between receivers within eachreference group, similar to the case using a single reference tag.

However, by allowing one or more receivers (i.e., receivers 100 c and100 d) to be included in two or more such reference groups (i.e., zones110 and 112), it is possible to dramatically improve system coveragewhile minimizing the total number of receivers and correspondinginfrastructure cost. As a specific example, assume that receivers i andk reside in one reference group, having reference tag T_(ik) whilereceivers j and k are in a second reference group, having a separatereference tag T_(jk). Receiver k acts as a “bridging receiver” forcomputing the counter offsets between receivers within the two separategroups as follows. The counter offsets ΔR_(ik) and ΔR_(jk) can bedirectly measured as described above. The counter offset betweenreceivers i and j, which lie in separate reference groups, is simplydetermined by the relationship

ΔR _(ij) =ΔR _(ik) −ΔR _(jk).  (5)

In similar fashion, all counter offsets between receivers within thesame or different reference groups can be determined. Note that, if areference tag signal is detected by a receiver not within the referencegroup assigned to this tag, the signal data can be discarded by theprocessing hub. This process can be readily extended to multiplereference groups by providing one or more bridging receivers for eachreference group pair.

In each “reference group,” a set of receivers may be assigned to onespecific reference tag. One can also define a “virtual group” ofreceivers in which a set of receivers is assigned to one specific zoneor field of coverage. The zone boundary, or field of coverage, for avirtual group is defined as a user-selectable region in which all activetags within the region are expected to have direct transmission paths toall receivers within the virtual group. Advantageously, a “virtualgroup” need not coincide with a “reference group”. Indeed, a virtualgroup may use a subset of receivers from within a reference group, ormay comprise receivers from more than one reference group. In the lattercase, a virtual group can be used to restrict the participation ofcertain receivers in a given computation, minimizing the probabilitythat “bad” data (i.e., data from indirect, or reflected paths) is usedin the computation of location.

The physical sub-regions or zones assigned to virtual groups can beoverlapped, next to each other, or even separated by some non-zerodistance. However, in the minimization of the error function (eq. 2),one limits the set of TDOA data to that data measured from receiverswithin the virtual group only.

Through a combination of “reference groups” and “virtual groups,” onecan significantly improve the scalability and flexibility of a precisionobject location system, expanding system capacity in terms of areacoverage while maintaining the same high degree of accuracy for tagposition measurements. The following examples illustrate these features.

Example Mosaic Receiver Pattern

In a single reference tag system, the area of coverage is essentiallylimited by the maximum range achievable from reference tag to thereceivers. This maximum range, in turn, is limited by regulatoryconstraints (e.g., FCC Part 15 rules for unlicensed radiators),blockages or attenuation from building materials and obstructions, tagplacement required to prevent erroneous returns from signal bounce offmetal walls, etc. If the maximum range achievable for any of the abovereasons is R_(max), then the coverage area is approximated by 2R_(max) ²as shown in FIG. 4. However, using a set of N reference tags (e.g., tags126, 127, 128, and 129) and a set of N reference groups (e.g., receivergroups respectively covering zones 120, 122, 124, and 126), the entiregroup of receivers of monitoring stations 130-138 may be arranged in apattern (mosaic or otherwise) that provides an expanded coverage area ofapproximately 2NR_(max) ², resulting in a greatly expanded area ofcoverage. Coverage area is scalable with any number of reference tagsand reference groups. Note that, unlike a set of four independent singlereference tag systems requiring sixteen receivers, the same coveragearea is handled using only nine receivers and three additional referencetags. This provides a significant cost reduction as the wiredinfrastructure costs are minimized and seven of the more costlyreceivers are replaced by three low cost reference tags. Furthermore,unlike the independently operating single reference tag systems witheach requiring its own hub processor, the multiple reference system mayutilize a single processor 139 connected to all receivers, as shown inFIG. 4.

It is noteworthy that the individual reference groups covering zones120, 122, 124, and 126 need not be physically next to each other as inthe mosaic configuration in FIG. 4. They may also be overlapped, or evenseparated by some distance, provided the connection (e.g., CAT-5 cables)between receivers 130-138 and processing hub 139 provide stable powerand clock signals, as well as reliable serial communications. Suchflexibility is very advantageous, for example, in the case that one ormore separated rooms or regions need to be monitored for tag locations.

Example Signal Blockage Problem

A limitation of a system using a single reference tag is possibleblockage of direct transmission paths between the reference tag and oneor more of the receivers. As noted above, a direct transmission pathfrom the reference tag to the receivers must be established in order togenerate a full matrix of all timing or counter offsets (ΔR_(ij)).However, due to the nature of an indoor environment, there may only be alimited number of such unobstructed, direct transmission paths. Forexample, walls, machinery, and other obstacles may create signalattenuation or even complete signal blockage from certain tag locations,negating the ability to use a single reference system or method. In FIG.5, for example, reference tag 201 has direct line-of-sight paths only tothree monitoring stations 141, 142, and 144 (i.e., UWB Receivers #1, 2and 4) while the transmission path to station 143 (Receiver #3) iscompletely blocked by an RF barrier 300 thereby preventing a singlereference tag design from working properly.

With a multiple reference system as shown in FIG. 5, another referencetag 202 is placed on the other side of the RF opaque barrier 300 so thatdirect line-of-sight paths become available to receivers #3 and #4 ofmonitoring stations 143 and 144. The set of receivers #1, #2 and #4 ofstations 141, 142 and 144 are then assigned to reference group 1 usingreference tag 201, and another set of receivers #3 and #4 of monitoringstations 143 and 144 are assigned to reference group 2 using referencetag 202. With reference tag 201, the timing offsets between receivers#1, #2 and #4 within reference group 1 may be directly measured usingequation (3). Similarly, the timing offset between receivers #3 and 4 ofreference group 2 may be determined using data from reference tag 202.Finally, using equation (5), the timing offset between the two groups ofreceivers can be determined as well, resulting in the unambiguousdetermination of a full matrix of timing offsets between all receivers.

Object tag positions within any of the zones can now be calculated. Forexample, object tag 301 can only be detected by receivers #1, #2 and #4,but this is sufficient to accurately determine its two-dimensionalposition. In similar fashion, object tag 302 can be detected byreceivers #1, #3 and #4, and object tag 303 can be detected by receivers#1, #2 and #3. Hence, the positions of each of these object tags can bereadily measured with the multiple reference tag system. This would nothave been possible using a single reference approach in the presence ofsignal blockage exemplified by RF barrier 300.

It is, of course, critical to accurately measure the time differences ofarrival of an RF signal between the different receiver sites. Any faultymeasurement of signal arrival time can result in a significant error incomputing object tag position. Note also that only the directtransmission path should be measured to determine the truetime-of-flight. This is implemented in the receiver by treating thefirst received signal as the direct transmit signal, since any reflectedsignals will arrive at later times. However, this approach has apotential drawback in a severe multipath environment, in the case whenonly bounced signals reach the receivers; i.e., no direct path exists.

For example, as illustrated in FIG. 6, the direct transmission pathbetween tag 301 and receiver #1 is blocked, but a bounced signal alongpath 147 (e.g. reflected by a side wall 149) may still reach thereceiver 151. Receiver 151 detects the reflected signal, and measuresits time-of-flight time, which would be larger than that from a signalpropagating along a direct transmission path from tag 301 to receiver151. This faulty measurement would result in a significant positioningerror.

To solve this problem, a virtual group technique supported by multiplereference tags may be used. Note that the virtual group technique may bebased on either reference tags and/or object tags. For a virtual groupassignment based upon reference tags 301 and 302, the user assigns agroup of receivers to each reference tag, making sure that each receiverwithin the reference group has a direct line-of-sight path to itsrespective reference tag.

Conversely, the virtual group technique can also be used when computingthe positions of object tags. That is, the field of interest issubdivided into multiple zones. Each such zone is defined by itsboundary and a virtual group of receivers which fully cover the zone. Inthis case, any tag within a particular zone must have a directline-of-sight path to the receivers within its virtual group. Note thatthis is a bit more complex than for virtual groups based upon referencetag as, in the latter case, only propagation paths from a single pointlocation need to be examined.

Tag positions are then determined based upon TDOA measurement data takenwithin each virtual group. If a tag signal is detected by more than onevirtual group, it may be possible to compute more than one position fora tag. In this case, the system can filter out redundant data using thefollowing criterion: If the resulting tag position determined by avirtual group is located outside of its zone boundary, the data ismarked as invalid and discarded. In effect, the zoning technique helpsto exclude the faulty arrival time data (of some receivers) from thecomputational process to minimize the error function. Thus, a highaccuracy tag position measurement can be achieved.

FIG. 6 illustrates how a virtual group technique may be used to minimizethe positional error under severe multipath conditions. Consider twovirtual groups for zones 1 and 2. The virtual group for Zone 1 includesreceivers #1, #2 and #4 of monitoring stations 151, 152, and 154, whilethe virtual group for Zone 2 includes receivers #2, #3 and #4 ofmonitoring stations 152, 153, and 154.

A reflected signal from tag 301 within Zone 2 might be detected byreceiver #1 within Zone 1, resulting in a faulty TDOA measurement atreceiver #1. Since one does not know a priori which zone the tagposition is in, the system will compute the tag positions from bothvirtual groups. However, the result from the first virtual groupcomputation would indicate that the tag position is outside of its zoneboundary (Zone 1). Consequently, the position data, which has a largeerror due to the faulty TDOA measurement, will be silently discardedbefore it is reported to the end user. On the other hand, thecomputation from the second virtual group including receiver #2, #3, and#4 would result in a position data within its zone boundary (Zone 2).This data, which does not have any faulty TDOA measurements from thevirtual group, is then reported to the user. As a result, only theposition data with high accuracy is reported.

Finally, in some applications, it may not be possible or desirable toplace a stationary reference tag within the field of interest. Themultiple reference tag approach shown in FIG. 7 encircles a field ofinterest by geometrically placing monitoring stations 161-164 andreference tags in a circular pattern. Alternatively, the field ofinterest may be partially encircled. In the illustrated example, allreference tags 401, 402, 403 and 404 are placed outside of the middle“monitored” region, with each reference tag being used to detect thetiming offsets between neighboring receivers. Again, using equation (5),the counter offsets between non-neighboring receivers can be determined,allowing precise measurement of location of any object tags locatedwithin the monitored area.

Embodiments described herein illustrate various ways to implement theapparatus and method aspects of the invention and are not intended tolimit the scope of the invention. It is known to those skilled in theart that mathematical and/or geometric relationships or equivalents,other than those equations illustrated herein, may be used to performthe necessary computations to determine the location of an object or toobtain timing offset information for the various timers of themonitoring stations. Geometric patterns other than those illustrated maybe used to arrange receivers and/or reference tags that define zones,and other mathematical relationships may be employed to compute orextrapolate timing offset between and among neighboring receivers. Thelayout of or geometric patterns of receivers and reference tags mayobviously be arranged to conform to the needs of actual deployment. Inaddition, free-running counters, delay lines, or other circuits knownnow or as may become known may measure or detect time-of-arrivalinformation to provide a computational basis to locate an object. Theinvention may be deployed utilizing a common clock source to synchronizethe receivers, or by deploying accurate independent clocks at multiplereceivers. Accordingly, the invention is not limited to the embodimentsor methods shown and described but instead, is defined by the appendedclaims.

What is claimed is:
 1. A method comprising: defining a first zone withina monitored area, a first group of receivers covering the first zone;defining a second zone within the monitored area, a second group ofreceivers covering the second zone; determining, via a processor, afirst position of a first tag based on timing measurements obtained viathe first group of receivers; determining, via the processor, whetherthe first position indicates that the first tag is within the firstzone; and when the first position indicates that the first tag is notwithin the first zone, not reporting data associated with the first tag.2. A method as defined in claim 1, further comprising, when the firstposition indicates that the first tag is within the first zone,reporting the data associated with the first tag.
 3. A method as definedin claim 2, wherein the data associated with the first tag includes atleast one of personnel information, transmitter identifier, packetnumber, or a material description.
 4. A method as defined in claim 2,wherein not reporting the data associated with the first tag comprisesdiscarding the data associated with the first tag, and wherein reportingthe data associated with the first tag comprises transmitting the dataassociated with the first tag.
 5. A method as defined in claim 1,wherein defining the first and second zones is based on a location of aradio frequency barrier in the monitored area.
 6. A method as defined inclaim 1, further comprising: determining a second position for a secondtag based on second timing measurements obtained via the second group ofreceivers; determining whether the second position indicates that thesecond tag is within the second zone; when the second position indicatesthat the second tag is not within the second zone, not reporting dataassociated with the second tag; and when the second location dataindicates that the second tag is within the second zone, reporting thedata associated with the second tag.
 7. A method as defined in claim 1,wherein each tag within the first zone has a direct line-of-sight pathto each of the first group of receivers, and each tag within the secondzone has a direct line-of-sign path to each of the second group ofreceivers.
 8. A method as defined in claim 1, wherein defining the firstzone includes establishing a first virtual boundary within the monitoredarea.
 9. A system comprising; a first group of receivers assigned to afirst zone within a monitored area; a second group of receivers assignedto a second zone within the monitored area; a processor to: determine afirst position for a first tag based on timing measurements obtained viathe first group of receivers; and discard data associated with the firsttag when the first position is in the second zone.
 10. A system asdefined in claim 9, wherein the processor is to report the dataassociated with the first tag when the first position is in the firstzone.
 11. A system as defined in claim 9, wherein the processor is to:determine a second position for a second tag based on second timingmeasurements obtained via the second group of receivers; and discarddata associated with the second tag when the second position is in thefirst zone.
 12. A system as defined in claim 11, wherein the processoris to report the data associated with the second tag when the secondposition is in the second zone.
 13. A system as defined in claim 9,wherein the first group of receivers is assigned to the first zone andthe second group of receivers is assigned to the second zone based on alocation of a radio frequency barrier in the monitored area.
 14. Asystem as defined in claim 9, wherein each tag within the first zone hasa direct line-of-sight path to each of the first group of receivers, andeach tag within the second zone has a direct line-of-sign path to eachof the second group of receivers.
 15. A system as defined in claim 9,wherein the first zone is defined by a first virtual boundary in themonitored area.
 16. An apparatus, comprising: memory comprising machinereadable instructions; and a processor to execute the machine readableinstruction to perform operations including: determining a firstposition of a first tag based on timing measurements obtained via afirst group of receivers assigned to a first zone defined in a monitoredarea, wherein a second group of receivers is assigned to a second zonedefined in the monitored area; and reporting information based on thefirst tag only when the first position is within the first zone.
 17. Anapparatus as defined in claim 16, wherein the operations include:determining a second position of a second tag based on timingmeasurements obtained via the second group of receivers; and reportinginformation based on the second tag only when the second position iswithin the second zone.
 18. An apparatus as defined in claim 16, whereinthe first and second zones are defined based on a radio frequencybarrier located in the monitored area.
 19. An apparatus as defined inclaim 16, wherein each tag within the first zone has a directline-of-sight path to each of the first group of receivers, and each tagwithin the second zone has a direct line-of-sign path to each of thesecond group of receivers.